1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system including the same, which is used as an optical system of optical equipment such as a video camera, a digital camera, and a television camera.
2. Description of the Related Art
As a method of reducing chromatic aberration of an optical system (lens system), there is known a method of combining different glass materials. Other than that, there is known a method of disposing a diffractive optical element having a diffraction action in a part of the optical system. It is known that the diffractive optical element is provided with an effect of chromatic aberration correction as well as an effect of an aspheric lens by changing a period of its periodical structure appropriately. Here, one light beam is still one light beam after being refracted, while one light beam is split into light beams of individual orders after being diffracted. Therefore, it is necessary to determine a grating structure so that light beams in a working wavelength range are concentrated in one specific order (hereinafter also referred to as a design order) when a diffractive optical element is used as the lens system.
Therefore, among diffractive optical elements that are used as a general diffraction lens, a diffractive optical element having a blaze structure can diffract light having a specific wavelength with high efficiency at a specific diffraction order. As a diffractive optical element having the blaze structure, there is known a diffractive optical element which diffracts light in a wide wavelength range with high efficiency (see Japanese Patent Application Laid-Open No. 2005-107298).
On the other hand, a light beam entering a grating side surface of a grating part of a diffraction grating behaves as being reflected or refracted by the grating side surface unlike the case of a grating surface, resulting in undesirable light. There is known a diffractive optical element in which a curvature of an envelopment surface of grating grooves and an angle of the grating side surface are optimized so that an incident light beam is hardly incident on the grating side surface in order to reduce undesirable light on the grating side surface (see U.S. Pat. No. 5,801,889).
The structure of changing an inclination of the grating side surface so as to reduce light having entered the grating side surface reaching an image plane (evaluation plane) in order that the undesirable light generated by the grating side surface is reduced does not always has a sufficient effect of reduction. It is preferred to set a grating thickness of the grating part of the diffraction grating to be small because when light that does not contribute to imaging enters a wall surface (grating side surface), a ratio of generation of undesirable light is small. Further, also concerning light that contributes to imaging, diffraction efficiency due to a variation of the light incident angle deteriorates less, which is desirable. It is known that in order to obtain high diffraction efficiency in a wide wavelength range in the diffractive optical element including diffraction gratings made of two different types of materials that are disposed in intimate contact to each other, a combination of a material having a high refractive index and low dispersion and a material having a low refractive index and high dispersion is necessary. In order to reduce the grating thickness of the grating part in the diffractive optical element including diffraction gratings made of two different types of materials that are disposed in intimate contact to each other, it is necessary to set a refractive index difference and a dispersion difference between the materials to be large. Therefore, it is necessary to prepare a material having a higher refractive index and lower dispersion, or a material having a lower refractive index and higher dispersion.
However, a relationship between the refractive index and the dispersion (wavelength dispersion of the refractive index) of an actually existing optical material generally shows a tendency that as the refractive index increases, the dispersion also increases. Therefore, it is very difficult to find a combination of multiple optical materials having a sufficient effect among currently used materials. There is known a diffractive optical element in which combinations of materials are increased so as to obtain relatively high diffraction efficiency in a wide wavelength range. However, in the structure, the entire grating thickness increases (the grating becomes thick). Therefore, undesirable light is generated when the light that does not contribute to imaging enters the wall surface (grating side surface), and it becomes very difficult to reduce the generated undesirable light.